1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an engine which controls an air-fuel ratio of the engine based on at least an output of an air-fuel ratio sensor disposed in the exhaust passage upstream of a three-way catalyst. More specifically, the present invention relates to such an air-fuel ratio control device that is able to detect a deterioration of the three-way catalyst based on at least an output of an air-fuel ratio sensor disposed in the exhaust passage downstream of the three-way catalyst.
2. Description of the Related Art
An air-fuel ratio control device for controlling an air-fuel ratio of an engine by a feedback control based on an output of one air-fuel ratio sensor (O.sub.2 sensor) disposed in an exhaust passage upstream of a catalytic converter is known as a single O.sub.2 sensor system. The single O.sub.2 sensor system is used for controlling the air-fuel ratio of the engine at a stoichiometric air-fuel ratio to improve the condition of the exhaust emissions by utilizing the ability of the three-way catalytic converter to a maximum degree.
Also, to compensate for the individual differences among cylinders or changes due to aging of the upstream O.sub.2 sensor, a double O.sub.2 sensor system using two O.sub.2 sensors has been developed (U.S. Pat. No. 4,739,614). In the double O.sub.2 sensor system, O.sub.2 sensors are disposed upstream and downstream of the catalytic converter in the exhaust passage, and the air-fuel ratio control is carried out based on the output of the downstream O.sub.2 sensor as well as the output of the upstream O.sub.2 sensor.
Nevertheless, even in the double O.sub.2 sensor system, if the catalyst in the catalytic converter is deteriorated, the condition of the exhaust emissions such as HC, CO, NOx is worsened, therefore it is necessary to detect the deterioration of the catalyst accurately.
To detect the deterioration of the catalyst in the catalytic converter, various methods or devices have been proposed
For example, in the system disclosed in U.S. Pat. No. 4,739,614, it is determined that the catalyst is deteriorated, when the ratio of the period T.sub.1 of the reversal of the output of the upstream O.sub.2 sensor to the period T.sub.2 of the reversal of the output of the downstream O.sub.2 sensor, i.e., T.sub.1 /T.sub.2, becomes larger than a predetermined value (or, alternatively, when the amplitude of the output of the downstream O.sub.2 sensor becomes larger than a predetermined value). However, in the above system, if the center value of the air-fuel ratio controlled by the air-fuel ratio feedback control deviates from the stoichiometric air-fuel ratio, the reversal of the output of the downstream O.sub.2 sensor does not occur. In this case, the output of the downstream O.sub.2 sensor oscillates at a small amplitude at a rich side or lean side of the air-fuel ratio.
Also, in some cases, the amplitude of the output of the downstream O.sub.2 sensor can become large even if the catalyst is not deteriorated. Further, if the downstream O.sub.2 sensor itself is deteriorated, the amplitude of the output of the downstream O.sub.2 sensor becomes smaller even if the air-fuel ratio of the exhaust gas downstream of the catalytic converter fluctuates largely.
Therefore, it is sometimes difficult to detect the deterioration of the catalyst accurately based on the period of the reversal or the amplitudes of the output of the O.sub.2 sensors.